Monolithic arrays of closely spaced, independently addressable semiconductor lasers are important light sources for applications such as optical disk recorders, laser printers, and fiber optic communication systems. With such laser arrays it is generally desirable to pack the laser elements as densely as possible. However, closely spaced laser elements are difficult to electrically connect and heatsink (cool). Furthermore, closely spaced laser elements tend to interact electrically, optically, and/or thermally. These interactions, called crosstalk, are usually undesirable.
A partial solution to the crosstalk problem is disclosed in U.S. Pat. No. 4,069,463, issued 17 Jan. 1978 to McGroddy et. al. and entitled "Injection Laser Array." That patent teaches mounting the laser elements in close electrical and thermal contact with a thermally and electrically conductive beam lead located in a substrate of insulating material. While the beam leads individually heatsink the laser elements, the beam leads are electrically isolated from each other. In the '463 patent, each laser element is defined and isolated from its neighbors by a groove through the epitaxial layers of the laser structure. Although the teachings of the '463 patent enable the fabrication of separately addressable and heatsinked laser elements, its successful implementation may be excessively complex.
Another partial solution to the crosstalk problems is taught in U.S. Pat. No. 4,531,217, issued 23 Jul. 1985 to Kitamura and entitled "Semiconductor Laser Device With Plural Light Sources." According to that patent, each laser element is mounted in close electrical and thermal contact with a common electrode on a homogeneous heatsinking submount. Physical separation of each laser element is provided by a groove through the substrate of the laser chip near the element's active layer. Each laser element is electrically addressed by individually contacting sections of the substrate between the grooves. Although this approach heatsinks the laser array, it may not provide adequate electrical isolation.
U.S. Pat. No. 4,916,710, issued 10 Apr. 1990 to Hattori and entitled "Multi-Point Emission Type Semiconductor Laser Device Therefor," teaches using a groove to electrically isolate laser elements from their neighbors. The groove cuts through the epitaxial layers of the laser structure and into a previously formed protrusion on the laser's substrate. Since the substrate is insulating, the groove isolates each laser element. Electrical contact to the substrate side of each laser element is achieved by removing the non-protruding regions of the substrate in order to access the first epitaxial layer, which is conducting. Electrical contact to the other side of each laser element is made with a single electrode that also serves as a heatsink. Although electrical isolation and heatsinking are both possible in a structure based upon the '710 patent, such a structure is believed to be rather complicated. In addition, since each contact on the substrate side must be large enough to accommodate a wire, the required distance between protrusions increases the minimum separation between the laser elements to a rather large value, especially if the laser array contains more than two elements. Finally, removing significant portions of the substrate, as is believed required in the subject structure, weakens the structural integrity of the laser chip, thus making it susceptible to breakage.
Another approach to alleviating the complexities of electrically addressing closely spaced elements in a diode laser array is disclosed in U.S. Pat. No. 4,870,652, issued 26 Sep. 1989 to Thornton and entitled "Monolithic High Density Arrays of Independently Addressable Semiconductor Laser Sources." In the '652 patent, the substrate side of the laser array is attached either to a submount or directly to a mounting surface of the final package. With the laser chip orientated as such, the separate electrical contacts on the epitaxial layers of each laser element remain exposed and accessible for individual electrical connections. However, this approach provides relatively poor heatsinking through the substrate, and therefore requires exceptional laser performance. While such performance has been obtained with laser elements fabricated by impurity induced layer disordering, it is not generally available with lasers utilizing other fabrication methods or designs. A consequence of the use of impurity induced disordering is that the state of the art currently restricts its use to wavelengths longer than about 750 nm. Such a wavelength limitation undesirably limits the uses of the laser arrays.
Accordingly, there is a need for designs and fabrication techniques which enable low crosstalk in closely spaced (dense) arrays of diode lasers. Beneficially those designs and fabrication techniques should be applicable to laser arrays having elements which emit light with wavelengths shorter than about 750 nm.